The TRMM Microwave Imager (TMI) is a nine-channel passive microwave radiometer which builds on the heritage of the Special
Sensor Microwave/Imager (SSM/I) instrument. It is flown aboard the Defense Meteorological Satellite Program (DMSP) platforms.
The TRMM Precipitation Radar (PR), the first of its kind in space, is an electronically scanning radar, operating at 13.8
GHz that measures the 3-D rainfall distribution over both land and ocean, and defines the layer depth of the precipitation.
The TRMM 3B31 Monthly 0.5 deg. x 0.5 deg. Combined Rainfall product, contains Rain rate, rain water, snow and graupel at 28
levels for a latitude band from 40 degree N to 40 degree S. It is derived from the TRMM combined algorithm (2B31), which combines
data from the TMI and PR to produce the best rain estimate for TRMM.

The current version of 3B31 only checks various quality flags in the 2A12, but not the 2B31. The recent version of 2B31 has
provided an uncertainty index. The quality check of 2B31 will be added in the next version to remove any input data with poor
quality. Pixels for which the 2A12 is either zero, or unreliable, currently do not allow the user to reconstruct the 2B31
original solution. This will be addressed in the next reprocessing. It may require a file format modification.